The invention relates to optical means for remotely monitoring physical and chemical parameters, and particularly for generating an optical signal based on colorimetric modulation of induced fluorescence.
The utility of optically based sensors over electrically based sensors for measuring physical and chemical parameters in harsh or inaccessible environments is well established, particularly when fluorometric sensors are coupled with fiber optics, Hirschfeld, "Remote Fiber Fluorometric Analysis," Energy and Technology Review, pgs. 17-21 (July 1980); Seitz, "Chemical Sensors Based on Fiber Optics," Analytical Chemistry, Vol. 56, pgs. 16(a)-34(a) (January 1984); Peterson and Vurek, "Fiber Optic Sensors for Biomedical Applications," Science, Vol. 224, pgs. 123-129 (Apr. 13, 1984); and Wickersheim and Alves, "Recent Advances in Optical Temperature Measurement," Industrial Research and Development (December 1979). Fiber optics are durable, corrosion-resistant, heat-resistant, and impervious to electrical or magnetic interference, and are available in very small diameters, which makes them amenable for use with miniature probes. Moreover, fluorometric probes are ideally suited for use with single fibers, wherein the same fiber is used to excite the fluorometric probe and to collect the emitted fluorescence. Single fibers can be used with fluorometric probes because the difference in wavelengths between the excitation beam and the fluorescence signal allows for ready separation of the signal from the beam even though they traverse the same fiber optic at the same time. While fluorometric probes are preferred over colorimetric probes for the foregoing reason, the number of reported colorimetric probes greatly exceeds that of fluorometric, e.g., Bishop, Indicators (Pergammon Press, New York, 1972). The lack of a wide selection of fluorometric probes can be a stumbling block for any particular use. Not only are fluorescent probes often not available to start with, but those that are available often fail to meet needed requirements, such as nontoxicity, chemical inertness, or thermal or photochemical stability.
Shaffar, in U.S. Pat. No. 4,495,293, "Fluorometric Assay," issued Jan. 22, 1985, discloses a chemical assay method comprising a reagent system including a fluorescent agent and a chromogenic agent, such that the emission band of the fluorescent agent overlaps the absorption band of the chromogenic reagent. The chromogenic agent is further selected so that its absorption characteristics are responsive to some sample molecule of interest, in that the degree of absorption by the chromogenic agent of fluorescence emitted by the fluorescent agent is related to the concentration of the sample molecule. The assay takes place in a liquid state so that fluorescent decay occurs both by radiative and by nonradiative mechanisms. For nonradiative mechanisms to become appreciable the concentration of the chromogenic molecule must be at least 2.times.10.sup.-3 molar (Parker, Photoluminescence of Solutions, Elsevier Publishing Company, New York, 1968, pgs. 83-85). At this concentration and above, many chromogenic molecules become opaque to the emissions of the fluorescent agents. Thus, for these chromogenic agents lower concentrations must be used which means that radiative decay of the excited states of the fluorescent molecules dominates, and less efficient absorption occurs. Shaffar's assay method also requires that the fluorescent agent be in direct chemical contact with the sample substance. Possible interaction between the sample substance and the fluorescent reagent greatly complicates the analysis of the optical signal generated by the assay procedure.
Energy transfer is employed in a class of immunoassay techniques which roughly are fluorescent analogs to radioimmunoassay. The techniques are exemplified by Maggio, U.S. Pat. No. 4,220,450, issued 2 Sept. 1980, entitled "Chemically Induced Fluorescence Immunoassay"; and Ullman et al., U.S. Pat. No. 4,174,384, issued 13 Nov. 1979, entitled "Fluorescence Quenching with Immunological Pairs in Immunoassays." Generally, the techniques include a stationary phase, e.g., a receptor which preferentially binds a particular ligand (the ligand being the compound whose concentration is to be determined) and a mobile phase, e.g., a ligand analog which competes with the ligand for binding sites on the receptor. A member of a fluorescer-quencher pair is covalently attached to the receptor, and the other member of the pair is covalently attached to the ligand analog. If the members of the fluorescer-quencher pair are within energy transfer distance of one another any fluorescence generated by the fluorescer is quenched. Thus, the amount of fluorescence emitted by a given number of fluorescer-labeled receptors is proportional to the number of binding sites occupied by ligand molecules relative to quencher-labeled ligand analogs. Conditions are established so that each quencher molecule reduces or quenches the fluorescence of a fluorescer molecule by the same amount.
The foregoing illustrates the limitations of the current sensor technology based on the interaction between fluorogenic and chromogenic substances. An alternative to available sensing methods which overcame some of these limitations would be highly advantageous, particularly for applications requiring monitoring of inaccessible, remote or hostile environments.